So yesterday I went out to a field going feral (I suspect someone put the plow to it some time back, but I don't think it has suffered from that for at least a few years). I dug up some dandelion roots to make dandelion root tea (as per Scout's instructions). I washed them in a bowl of water and when I finished, I emptied the bowl of sediment. But remaining in the bowl was a fine black powder. When I put water in and swished it around, the powder didn't move much, which tells me it's heavy. Does this sound like lead? Or could it be something as innocuous as iron? The field sits far from any roads, but a railroad runs right by it (which I foolishly did not consider when scoping out dandelions).

Try picking up the particles with a magnet, and if they adhere, they are iron. It's unlikely you will find lead in pure form as small particles as you described, and of course, neither lead nor creosote would adhere to a magnet.

I should have mentioned that lead poisoning usually occurs from ingesting old leaded paint or dirt from places previously exposed to leaded fuel emissions or spills, such as old vacant lots, or old lead water pipes that contaminate drinking water. Plants do not absorb it, so washing the roots with clean water as you did, should ensure their safety, at least from lead. Other heavy particles that fall to the bottom as you described that perhaps won't adhere to a magnet may be silicates, mica or other normal soil material, but if you can see them, they won't be lead, because lead contamination is microscopic. I worry more about contaminants such as herbicides and pesticides that one cannot see, but these do deteriorate over time, so an old, abandoned field that has not suffered these indignities for a few years, should be relatively safe to collect roots in.